1. Field of the Invention
The present invention relates in general to an automatic transmission, and more particularly to a vehicle automatic transmission having a plurality of groups of operating positions each of which consists of a plurality of successive operating positions and which are selected depending upon a running condition of a vehicle, so that the automatic transmission is controlled to be a selected one of the successive operating positions of the selected group.
2. Description of the Related Art
For vehicles, there have been widely used various types of automatic transmission having a plurality of planetary gear sets, clutches and brakes. One of these types of automatic transmission is a multiple-step transmission having at least seven forward-drive positions. JP-2003-130152A discloses a typical example of this type of automatic transmission, which has
(a) a first input path for transmitting a rotary motion of an input member at a predetermined first speed ratio, (b) a second input path for transmitting the rotary motion of the input member at a predetermined second speed ratio higher than the first speed ratio, such that the speed of the rotary motion received from the input member is lowered during transmission of the rotary motion through the second input path, and (c) a primary shifting portion having a total of seven or eight forward-drive positions and including an output member, three planetary gear sets, a plurality of clutches, and a plurality of brakes. The planetary gear sets have a plurality of rotary elements which include an output rotary element connected to the output member and which are connected to each other such that the rotary elements are rotated relative to each other in predetermined relationships with each other. The clutches are selectively engaged to connect selected ones of the rotary elements to the first or second input path, and the brakes are selectively engaged to hold selected ones of the rotary elements in a stationary state, so that a selected one of the operating positions is established by controlling engaging and releasing actions of the clutches and brakes, whereby the rotary motion of the output rotary element connected to the output member is transmitted from the primary shifting portion through the output member.
Similar types of automatic transmission are disclosed in JP-2002-206601 A, JP-8-105496A, JP-2000-199549A, JP-2000-266138A, JP-2001-82555A, JP-2002-227940A, JP-2002-295609A, and JP-2956173B2.
The conventional types of automatic transmission described above are generally shifted on the basis of running condition parameters of the vehicle such as the running speed and the operating amount of an accelerator pedal of the vehicle, and according to predetermined shifting conditions represented by a plurality of suitable shift-up and shift-down maps, which correspond to respective different running modes or conditions of the vehicle such as a sporty-running mode or a snow-running mode, which are desired by an operator of the vehicle. The shift-up and shift-down maps represent predetermined relationships between the running condition parameters and the operating positions of the automatic transmission. However, the number of operating positions available, the speed ratios of the operating positions, stepping ratios (ratios of the speed ratios of the adjacent operating positions), and an overall range of the speed ratio (so-called “spread”, which is a ratio of the maximum speed ratio to the minimum speed ratio of the transmission) are the same for all of the shift-up and shift-down maps corresponding to the different running modes. In this respect, the shifting of the automatic transmission cannot be controlled so as to sufficiently satisfy the operator's desire regarding the running mode or condition of the vehicle. Although the automatic transmission can be manually shifted up and down by the vehicle operator, this manual control is not completely satisfactory to the operator, since the individual operating positions have the respective fixed speed ratios.
The automatic transmission may be given a plurality of groups of operating positions, each group consisting of a plurality of operating positions having respective different speed ratios, by increasing the numbers of the planetary gear sets, clutches and brakes. In this case, one of the groups is selected depending upon the running mode or condition of the vehicle desired by the operator. While this type of automatic transmission is improved in the versatility of its control, the automatic transmission tends to be complicated in construction with increased numbers of the planetary gear sets, clutches and brakes, and suffer from an increased cost of manufacture and difficulty in installation of the automatic transmission on the vehicle.